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Lineaments and the Separation of Sicily Island from the Chalk Hills by the Ouachita River Valley, Northern Catahoula Parish, Louisiana*
By
Richard P. McCulloh1
Search and Discovery Article #50005 (2003)
*Adapted from Louisiana Geological Survey News, v. 13, no. 1, June, 2003, p. 1-2. Appreciation is expressed to the author and the Louisiana Geological Survey, Chacko J. John, Director and State Geologist.
1Louisiana Geological Survey ([email protected])
Background and General Statement
In a brief section
entitled, “Diversion of the Ouachita River near Harrisonburg, La.,” of the
Report of 1905 of the Geological Survey of Louisiana (p. 303-304; includes “Fig.
24” on facing page 302), A.C. Veatch (1906) addressed the origin and history of
the current course of the Ouachita River 
flood
plain relative to Sicily Island
and the Chalk Hills. He set forth the case that the present course was a result
of constructional depositional dynamics at the confluence of the Ouachita and
Mississippi flood plains in early Quaternary time. During glaciation, he argued,
outwash deposition raised the level of the flood plain of the Mississippi and
the distal reaches of the flood plains of its tributaries at their confluences
with it. At this time the southward-flowing Ouachita ran north and east of
Sicily Island. As a result of the voluminous outwash deposition in the
Mississippi flood plain, the gap between Sicily Island and the mainland
encompassing the Chalk Hills to the west became buried, and was occupied by a
flood plain contiguous with that of the Mississippi. This gap was raised as much
as 18 m (60 ft) above present stream bottoms, yet was likely somewhat lower than
the Mississippi flood plain proper because the depositional cone advancing down
the Mississippi course must have been quite large compared to the volume of
sediment being transported by the Ouachita system. This difference in elevation
made possible the shifting of the Ouachita course into the gap before the onset
of the succeeding period of downcutting that sculpted the outwash deposits into
the terraces now preserved to the east and north of Sicily Island. The above
history inferred by Veatch was his way of explaining the presence of the
Catahoula Shoals in the Ouachita River to the west of Sicily Island—after the
river shifted and had begun cutting down (Figure 1),
it encountered a preexisting low drainage divide between minor north-flowing and
south-flowing drainages that had formerly occupied the gap (Figure
2), at the position marked by the shoals. The question, however, remains:
what, if anything, originally accounted for the gap itself between Sicily Island
and the main region of hills to the west?
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In his extensive monograph on the Lower Mississippi Valley, Fisk (1944) included a short section on interpreted lineaments, which he referred to individually as fault zones and collectively as a regional fracture pattern. It appears that Fisk and his team were working with small-scale black-and-white aerial photography, and traced drainage lineaments discernible on that imagery to come up with the trends. The lineaments resolve as a single pair of nearly orthogonal sets, oriented NE-SW and NW-SE, forming a rectilinear grid in the Mississippi embayment. This appears to have been the earliest such interpretation in Louisiana. A plot of Fisk’s data by Gay (1973) showed two very tight trends with mean orientations of N39oW and N49oE.
A digital shaded-relief rendering of
Louisiana topography (Figure 3) gives a
clear suggestion of two intersecting lineaments, oriented approximately
N37oW and N51oE, defining the large
eastward-projecting promontory that encompasses the Chalk Hills west of
Sicily Island. The apparent lineaments follow the pre-Holocene/Holocene
contact along the western valley wall of the Ouachita River
ReferencesChawner, W.D., 1936, Geology of Catahoula and Concordia parishes: Department of Conservation, Louisiana Geological Survey, Geological bulletin no. 9, 232 p. plus plates (includes one 1:62,500-scale geologic map). Fisk, H.N., 1938, Geology of Grant and La Salle parishes: Department of Conservation, Louisiana Geological Survey, Geological bulletin no. 10, 246 p. plus plates (includes two 1:62,500-scale geologic maps). Fisk, H.N., 1944, Geological investigation of the alluvial valley of the lower Mississippi River: Vicksburg, Mississippi, U.S. Army Corps of Engineers, 78 p. plus plates. Gay, S.P., Jr., 1973, Pervasive orthogonal fracturing in earth’s continental crust: Tech. Publication No. 2, Salt Lake City, American Stereo Map Company, 121 p. plus appendices. Huner, J., Jr., 1939, Geology of Caldwell and Winn parishes: Department of Conservation, Louisiana Geological Survey, Geological bulletin no. 15, 356 p. plus plates (includes two 1:62,500-scale geologic maps). Snead, J.I., and R.P. McCulloh (compilers), 1984, Geologic map of Louisiana: Baton Rouge, Louisiana Department of Natural Resources, Louisiana Geological Survey, scale 1:500,000. Veatch, A.C., 1906, Geology and underground water resources of northern Louisiana, with notes on adjoining districts, in Geological Survey of Louisiana, Report of 1905: Louisiana State Experiment Station, Geological Survey of Louisiana, Bulletin 4, p. 249–457. |
Figure
1. “Change in Ouachita River drainage near Harrisonburg, La.,”
from Veatch (1906, p. 302, his figure 24).
Figure
2. “Diagram illustrating the deflection of Ouachita River and the cause
of the formation of the Catahoula Shoals near Harrisonburg, La.,”
redrawn from Veatch (1906, p. 303, his figure 25).
Figure3.
Louisiana digital topography, annotated to show features discussed in
text (heavier lines = interpreted lineaments; modified from Chalk Butte
Inc., 1994).